1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of fabricating a capacitor that can improve the quality of the inter-electrode dielectric layer and reduce the electrical resistance of the lower electrode to increase the Q-factor of the capacitor.
2. Description of the Related Art
In the prior art, a capacitor compatible to MOS process usually has a metal-insulator-metal (MIM) structure or metal-insulator-silicon (MIS) structure. However, a MIM or MIS capacitor suitable to high-voltage applications is usually insufficient in the capacitance per unit area. Meanwhile, for mixed-mode devices or RF devices including capacitors, it is frequently required to save a large area of the die for the capacitors to achieve sufficient capacitance satisfying the design rule. However, after the semiconductor industry advanced to deep sub-micron generation, increasing the capacitor area will reduce the integration degree of devices more significantly so that the economic effect of manufacture is lowered more. Therefore, increasing the unit-area capacitance of the capacitors is always desired in the semiconductor industry.
In view of the foregoing, a capacitor fabricating method integrated with MOS process is proposed. In the method, a doped region is formed in a semiconductor substrate as a lower electrode, and then an oxide layer as the dielectric layer of the capacitor is grown on the doped region simultaneously with the gate oxide layers in the MOS area on the same die. A polysilicon layer is formed covering the capacitor dielectric layer and the gate oxide layer, and is then patterned into gates and an upper electrode of the capacitor.
Nevertheless, since the dielectric layer is formed after the doping step for forming the lower electrode, the quality thereof is not good. Meanwhile, the dopant concentration in the doped region is not so uniform. Moreover, the lower electrode constituted of a doped region has a higher resistance than metal, so that the Q-factor of the capacitor is usually too small to satisfy high-frequency applications.